(Not Applicable)
The invention generally relates to compositions having active oxygen scavenging capacity and the use of these compositions for improved packaging of oxygen sensitive substances. Formulations are disclosed which may be fabricated into packaging articles or used as container liners/coatings.
Plastic materials have continued to make significant advancements into the packaging industry due to the design flexibility of their material and their ability to be fabricated in various sizes and shapes commonly used in the packaging industry. The deployment of plastic materials into packaging articles such as films, trays, bottles, cups, bowls, coatings and liners is already commonplace in the packaging industry. Although plastic materials offer the packaging industry many benefits with an unlimited degree of design flexibility, the utility of plastic materials has remained inhibited in situations where barrier properties to atmospheric gases (primarily oxygen) are necessary to assure adequate product shelf life. When compared to traditional packaging materials such as glass, steel or aluminum, plastics offer inferior barrier properties which limits their acceptability for use in packaging items that are sensitive to atmospheric gases, particularly when the exposure to the atmospheric gases will entail extended time periods. The packaging industry continues to seek packaging materials which offer the design flexibility of plastics and at the same time have the barrier properties of glass, steel or aluminum.
It should be recognized that there are two broad types of barriers for shielding packaged oxygen sensitive substances from oxygen (generally oxygen from air). One is known as a passive oxygen barrier and finds utility because of superior resistance to the permeation of oxygen through such constructions. Glass and metal are essentially perfect passive oxygen barriers. Condensation polymers, particular polyesters such as polyethylene terephthalate (PET) have found wide acceptance in the packaging industry and are moderately good passive oxygen barriers. Polyamides, such as polyhexamethyleneadipamide and polyphthalamides, are generally better passive oxygen barriers than polyesters when deployed in similar constructions.
The other type of oxygen barrier is known as an active oxygen barrier. An active oxygen barrier is a substance capable of intercepting and scavenging oxygen (by undergoing chemical reaction with the oxygen), for example, as the oxygen attempts to permeate through the packaging. A major salient feature of active oxygen scavengers is their ability not only to intercept oxygen from air as it attempts to reach the package cavity but also to provide the means to eliminate unwanted oxygen (often called head space oxygen) from within the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. Only active oxygen scavengers can remove unwanted oxygen from the package cavity. Active oxygen scavenging implies, therefore, consumption of a material incorporated in the package. The material is progressively consumed so that the active oxygen scavenging ability is eventually depleted or at least diminished. However, this eventual depletion of the active oxygen scavenging moiety can be adjusted so that the depletion occurs only well after the required oxygen free shelf life of the packaged product which is typically one year or less.
Active oxygen scavengers are known and have been used in a variety of constructions. Optimally, active oxygen scavengers should have as many as possible, or at least some, of the features recited below:
(1) Their oxygen scavenging ability should be present both in the absence and/or in the presence of water or moisture.
(2) They should have clarity similar to that of PET or other packaging thermoplastics when necessary for production of clear bottles or films.
(3) They should be self-adherent to adjacent layer(s) when used as layer(s) in a multi-layer package construction.
(4) They should be evenly dispersed throughout the package so as to afford optimum and uniform opportunity to scavenge oxygen.
(5) They should have glass transition temperatures above filling and storage temperatures (at least above about 30xc2x0 C.) so that they exist as solids or solid films for these purposes.
(6) When used as a container liner, they should be capable of being sprayed onto the inner surface of a container from an aqueous system (as opposed to a lacquer which would require evaporation of organic solvents).
(7) The decomposition products to which the active oxygen scavengers are decomposed after reaction with oxygen must not be deleterious to the packaged product or must be shielded from the packaged product.
(8) The mechanism of their reaction with oxygen must not be deleterious to the strength, clarity, or other salient features of the packaging article.
What is needed are active oxygen barrier materials possessing as many as possible of the features recited above, which may be produced at reasonable cost, and which have sufficient oxygen scavenging and barrier properties to offer the possibility of target shelf lives in the range of six months to two years for oxygen sensitive products. This invention further addresses such need.
In a commonly assigned, related, and co-pending US application filed on Sep. 23, 1996 and having Ser. No. 08/717,370, now U.S. Pat. No. 6,083,585 it was disclosed that certain hydrocarbons, such as polyolefins, (especially polydienes) when present in small amounts as polyolefin oligomer blocks in a block copolyester polymer added substantial active oxygen scavenging capacity to packaging polyesters which showed no active oxygen scavenging capacity what-so-ever in the absence of the polyolefin oligomer blocks. The oxygen scavenging copolyesters of the above-referenced application were comprised predominantly of packaging polyester segments with only an oxygen scavenging amount of polyolefin oligomer segments present to supply the oxygen scavenging capacity required for the intended packaging application. The copolyesters of the application having Ser. No. 08/717,370 now U.S. Pat. No. 6,083,585 were typically in the range of about 0.5-12 wt % polyolefin oligomer segments with the remainder comprising polyester segments. An especially preferred embodiment was a copolyester of about 4 wt % polyolefin oligomer segments with the remainder being polyester segments. Such block copolyesters comprising low weight percent levels of polyolefin oligomer segments have properties (such as melting point, viscosity, and clarity) very similar to the unmodified polyester from which the polyester segments were derived. In particular, layers in laminar packages and bottles having one or several layers of unmodified polyester and one or several layers of oxygen scavenging block copolyester as described above, were self-adherent and packaging articles appeared to be a monolithic (rather than layered) construction.
In related PCT Application Number PCT/US98/02991 which was filed on Feb. 17, 1998 the concept of implanting high capacity oxygen scavenging polyolefin oligomer segments was extended to polyamides. The above referenced PCT application disclosed block copolyamides comprising predominantly polyamide segments and an oxygen scavenging amount of polyolefin oligomer segments. As was the case for the earlier disclosed copolyesters, the corresponding copolyamides had properties very similar to unmodified polyamides, especially the polyamide from which the polyamide segments were derived. Polyamides are generally considered to be an inherently superior passive barrier as compared to polyesters. Thus copolyamides not only had substantial active oxygen scavenging capacity but also had improved passive barrier properties since they were comprised mainly of polyamide segments. In this application, additional oxygen scavenging moieties are disclosed which when incorporated into copolyesters or copolyamides exhibit the superior active oxygen scavenging capacity as was demonstrated in related prior applications. Also disclosed is the incorporation of active oxygen scavenging moieties into other polycondensates in addition to copolyamides and copolyesters. Further disclosed is the incorporation of active oxygen scavenging moieties into addition type (as opposed to condensation type) copolymers. Another embodiment of this application involves the incorporation of the previously disclosed oxygen scavenging moieties (such as polybutadiene oligomer) into an aqueous based spray formulation for use as a container liner/coating for removal of head space oxygen from canned goods via active oxygen scavenging.
In earlier and related patent applications previously cited above, it was disclosed that certain hydrocarbon materials could be adapted for deployment as active oxygen scavengers in packaging articles. These active oxygen scavengers when placed in the walls of a packaging article would intercept and react with oxygen (from air) as it attempted to pass through the package wall thereby shielding the package contents from oxygen and extending the useful shelf life of the packaged oxygen sensitive substance. When used in packaging, active oxygen scavengers can also react with and remove head space oxygen from the package cavity provided that means exist for oxygen in the package cavity to contact and react with the active oxygen scavenger. The ability of hydrocarbons to react with oxygen is well known in the art and began to attract research attention in the early twentieth century because of the unwanted degradation of such materials as automobile tires and vegetable oils. Eventually, it was recognized that the propensity of hydrocarbons to oxidize could be used to advantage in packaging when deployed as active oxygen scavengers. However, it was necessary to overcome two major obstacles in order to reduce the phenomenon to practice. First, it was necessary to identify those hydrocarbons which were reasonable to use from an economic view but which also had sufficient oxygen scavenging capacity to provide the desired shelf life. Secondly, it was necessary to find a way to innocuously incorporate these materials into modem packaging articles which could be fabricated using current state-of-the-art packaging equipment. Other considerations included clarity of the package and fitness for recycle of the package. These issues were addressed and largely resolved in earlier and related patent applications previously cited above.
In those earlier and related applications, it was disclosed that hydrocarbons such as polyolefin oligomers had sufficient commercial oxygen scavenging capacity to extend shelf life of oxygen sensitive products. Especially effective is polybutadiene oligomer. It is not fully understood whether or not this effectiveness is because carbon to carbon double bonds (olefinic unsaturation) are present in polybutadiene oligomer. It was also disclosed that the oligomers could be functionally terminated with a chemical group capable of entering into polycondensation reactions. The functionally terminated polyolefin oligomers were then incorporated as blocks in a polycondensate. The copolycondensates, such as copolyesters and copolyamides, were extremely compatible with commonly used packaging polycondensates and as such amenable for use in packaging articles. In this application, this concept has been extended to the use of a polyether oligomer (specifically polypropylene oxide) as the oxygen scavenging moiety. There is no olefinic unsaturation what-so-ever in polypropylene oxide oligomer. While not wishing to be bound by theory, it is Applicants"" belief that oxygen scavenging occurs not only at the xe2x80x94CH2xe2x80x94 sites in polypropylene oxide but also at the xe2x80x94Oxe2x80x94 ether sites.
In order to incorporate the polypropylene oxide oligomer into a packaging copolycondensate, it was first necessary to add terminal functional groups capable of entering into polycondensation to the polypropylene oxide oligomers. Subsequently it was possible to form copolycondensates, such as copolyesters and copolyamides, having polypropylene oxide oligomer segments. The weight percent of polypropylene oxide oligomer segments, the molecular weight of the polypropylene oxide oligomers, and the average diameter size of the polypropylene oxide oligomer segments had to be determined to achieve optimum compatibility, clarity and scavenging capacity as was the case for copolycondensates having polyolefin oligomer segments. These polypropylene oxide oligomer containing copolycondensates are typically deployed as at least one layer in the wall of a multi-layer packaging article.
U.S. Pat. No. 5,605,996 (Chuu et al.) discloses the use of propylene oxide rubber as an oxygen scavenger but requires the presence of both olefinic unsaturation and moisture to function as an oxygen scavenger. Applicants"" copolycondensates having polypropylene oxide oligomer blocks contain no olefinic unsaturation and scavenge oxygen either in the absence or presence of water (moisture) when promoted with suitable catalyst. U.S. Pat. No. 5,529,833 (Speer et al.) discloses a multi-layer oxygen scavenging structure wherein at least one layer consists essentially of an ethylenically unsaturated hydrocarbon. As noted above, applicants"" copolycondensates having polypropylene oxide oligomer blocks contain no olefinic unsaturation.
The prior related applications cited above are all directed to compositions which comprise copolymers of condensation polymers, especially polyesters and polyamides. The copolymers are active oxygen scavengers because blocks of segments comprising oxygen scavenging moieties have been implanted into the copolycondensates. In the prior related applications, Applicants have disclosed the use of polypropylene, poly(4-methyl)1-pentene and polybutadiene as oxygen scavenging moieties which are effective when included in a polycondensate. In this application, the use of polypropylene oxide oligomer has been disclosed as an oxygen scavenging moiety. While many embodiments have been disclosed involving the use of the previously disclosed oxygen scavenging copolycondensates, compatibility with package construction is optimal when the oxygen scavenging copolycondensates are used in polycondensate based packages. For example, the oxygen scavenging copolyesters are most compatible when used with adjacent layers of packaging polyester. In a similar manner, the oxygen scavenging copolyamides are most compatible when used with adjacent layers of packaging polyamide. While packaging articles based on polycondensates are very common, there still exists a wide variety of applications for packaging articles based on addition type polymers.
In several embodiments of this invention, applicants have extended the concept of incorporation of high oxygen scavenger capacity moieties into addition type polymers so as to create addition type oxygen scavenging copolymers. These oxygen scavenging addition type copolymers may be used in any suitable embodiment but are intended primarily for use in addition polymer based packaging articles such as those comprising polyolefins including polypropylene, polyethylene, and mixtures of the preceding. As was done for polycondensates, Applicants disclose the use of polyolefin oligomers, preferably polypropylene, poly(4-methyl)1-pentene, polybutadiene, and also the use of polypropylene oxide as preferred oxygen scavenging moieties which are effective scavengers when included in polyaddition polymers. Further, Applicants preferred method of preparation is by transesterification of pre-made polyaddition polymers. Some addition polymers may already have esterification reaction sites on the polymer backbone, for example those comprised of acrylic acid or acrylic acid derivatives. Of course many polyaddition polymers, such as polyolefins, have no esterification sites. In such instances, it is generally necessary to treat the addition polymer with a substance which can add the needed esterification sites to the addition polymers. A preferred class of reactants for such purposes is an unsaturated acid, its anhydride, or derivatives thereof. Maleic anhydride (or derivatives of it) is especially preferred and such a process is generally well known in the art as maleation.
For further understanding it may be useful to consider Formulas I and II below:
I. Hxe2x80x94Oxe2x80x94(OSM)xe2x80x94Oxe2x80x94H
II. H2Nxe2x80x94(OSM)xe2x80x94NH2
In Formulas I and II, OSM represents a divalent oxygen scavenging moiety such as polypropylene oxide or the other scavenging moieties recited above. In Formula I the dihydroxy functionally terminated form of OSM is shown and Formula II the diamino functionally terminated form of OSM is shown. The OSM may be singly functionalized or may be functionalized to a degree greater than two, but double functionality is shown in Formulas I and II as one of many possible degrees of functionality. Also, other functional groups attached to the OSM are possible and suitable for the purposes of this invention, but only hydroxy and amino are shown for the sake of explanation and illustration. It will be obvious to those of ordinary skill in the art that the entities represented in Formulas I and II are capable of entering into polycondensation and/or transesterification reactions. In this invention, Applicants react species of Formulas I or II with addition polymers which have acid sites (or other suitable reaction sites) and incorporate the OSM into the addition polymer by condensation or esterification. The net result affords a simple and direct method for adding a precise amount of oxygen scavenging capacity in the form of the various OSM""s recited above to an addition polymer.
Generally, the copolyaddition polymers will be comprised predominantly of polyaddition segments and have only enough OSM segments to provide the required oxygen scavenging capacity for the planned application. Predominantly, in this sense, is defined as over 50 wt % polyaddition segments in the addition type oxygen scavenging copolymers. In practice, the copolyaddition oxygen scavengers will be comprised of OSM segments in the range of 0.5 to 12 wt % of the scavenging copolymers. Preferably the OSM segments will comprise from about 2 to about 8 wt % of the copolymer, and most preferably from about 2 to about 6 wt % of the copolymer. As was the case for oxygen scavenging copolycondensates, it is desirable to use only the minimally required amount of OSM segments so that the oxygen scavenging addition copolymer has properties very similar to unmodified addition polymers especially the addition polymer from which its segments were derived. A PCT Patent Application (Ching et al.) published on Dec. 19, 1996 and designated as WO 96/40799 discloses a polyethylenic polymer having a backbone with esterification/transesterification sites located thereon and methods of esterification of said sites. The Ching et al. patent further discloses attachment (at the active sites) on the polymer of pendant groups which have a carbon atom having an attached hydrogen atom wherein said carbon atom is adjacent to a list of moieties as further recited in the Ching et al. disclosure. In practice, the Ching et al. reference ultimately discloses a composition comprising a transition metal and a modified polyethylenic polymer capable of scavenging in the range of about 40-63 cc of oxygen per gram of composition after 28 days. The oxygen scavenging copolyaddition polymers disclosed by Applicants are easily distinguished from the Ching et al. disclosure in that Applicants"" copolymers are capable of scavenging about 83 cc of oxygen per gram of copolymer in 28 days under similar conditions, even in the absence of transition metal catalyst
In another embodiment of this invention, Applicants disclose a method for adding active oxygen scavenging capacity to widely used container coating compositions. It is a common practice in the packaging industry to use very thin plastic coatings on the inside of metal (iron or aluminum) can surfaces to prevent acidic food and beverages from corrosive attach and associated contamination with ionic metals. Particularly acute is the situation for canned carbonated beverages, such as beer and soda pop, where the dissolved carbon dioxide assures a very acidic and corrosive condition. In addition to corrosion prevention, another desirable attribute for can coating is the ability to remove unwanted oxygen from the package cavity where in such oxygen was inadvertently introduced during filling of the container. In can coatings, there is little concern with regard to oxygen which may enter the package cavity from outside since the can metal is essentially a perfect passive oxygen barrier to permeation of outside oxygen. For packaging of beer in cans, current technology is capable of placing beer in sealed cans at oxygen levels as low as about 200 PPB. Pasteurization of the beer may further reduce the oxygen level to as low as 100 PPB which remains in the can to react with and deteriorate the beer stored in the metal can. The taste aspects of beer are highly dependent upon reaction with trace amounts of oxygen. Further reduction of the amount of head space oxygen in a beer can provides the means for better tasting beer and/or longer shelf life for beer packaged in cans, hence the need for active oxygen scavengers in can coating plastics.
Some of the most commonly used can coatings are epoxy-amine-acrylate (EAA) coatings which are sprayed on to an unfilled metal cup (i.e., a can which has not yet had the top placed on it) as a water borne composition prior to a short cure of about 2 minutes at about 200xc2x0 C. Later, similarly coated and cured can tops are applied to complete the package. In all instances, curing of a water borne spray is more environmentally friendly than evaporation of organic solvents from a lacquer. For coating cans intended for use with food, beverages, and comestible products in general, the advantages of working with a water borne spray coating versus and organic solvent spray (lacquer) are even more pronounced. Applicants in this invention disclose a method for incorporation of the species of Formulas I and II above into a water based can coating emulsion thereby adding oxygen scavenging capacity to can lining formed therefrom. A PCT Patent Application (Bansleben et al.) published on Sep. 12, 1997 and designated as WO 97/32925 discloses active oxygen scavenging can coatings. However, the Bansleben et al. reference discloses only the use of an oxygen scavenging xe2x80x9clacquerxe2x80x9d which may be used as a coating on cans and other rigid containers. While there are other major differences, Applicants can coatings are easily distinguished from the Bansleben et al. coating in that they are formed from a water based emulsion and applied to the can as a water borne spray as opposed to a lacquer.